1. Field of the Invention
The present invention is related to the maintenance of communications networks. More particularly, the invention is directed to improved monitoring of transmissions errors in telecommunications networks.
2. Discussion of Related Art
It has been known to provide synchronous-transfer modes (STM) control of traffic using frames for such services as T-1, T-3 and others. More recently, hybrid ATM/TDM networks provide synchronous-transfer mode (STM) control of traffic from both the ATM and TDM payload sources. They permit a graceful, piecewise implementation of the megabits-per-second speeds of asynchronous transfer mode (ATM) packet-routing segments within existing voice, fax and data-communications networks having conventional digitally-switched call circuits (DSCs) that carry STM communications. STM communications use a signal-reference frame that provides dedicated time slots for the time-division multiplexing (TDM) of signals passing over the individual links in an STM network.
In theory, it should be possible to use the TDM trouble shooting criteria for monitoring the quality of hybrid ATM/TDM service and obtain the level of quality and efficiency achieved in purely TDM networks. However, field experiments that attempted real-time detection of transmission errors in call circuits combining ATM and TDM payloads resulted in imprecise descriptions of the troubles. The potential throughput efficiency advantages of the hybrid ATM/TDM systems cannot be realized so long as false-positive trouble flags mask the true nature and location of trouble spots in a network.
Even for purely TDM payloads, the conventional reporting of transmission errors as xe2x80x9cerrored secondsxe2x80x9d, does not accurately reflect the condition of the data actually received. The errored seconds are measured using parity bit (PB) or cyclic redundancy check (CRC) analysis, but they are determined using a fixed frame clock. In this time-locked conventional error reporting scheme, out-of-frame format errors (OOF) preempt the results of parity and CRC tests, that is, every frame transmitted while an OOF condition exists is a continuation of the transmission error, regardless of the accuracy of the data within that frame. When both the PB and CRC errors and the OOF errors are reported together as a total length of time, the entire time elapsed before the expected frame format reappears with the correct intra-frame parity or redundancy, is reported in those xe2x80x9cerroredxe2x80x9d or xe2x80x9cOOFxe2x80x9d seconds.
Conventional TDM service profiles reporting OOF xe2x80x9cerrored secondsxe2x80x9d are compiled by terminal equipment such as the DDM-1000 or the DCS 3/1, or an interface unit (IU) that is an adjunct to such terminal equipment.
Over-reporting of transmission errors is particularly serious for the digital service providers (DSPs) who are subject to tariff regulations. Often times the detection of transmission errors fails to distinguish between the types of errors. This distinction could be important since a DSP must be prepared to defend its performance as a licensee by proving the quality of their service and the reliability of the service that they have provided to the public under their operating licenses.
Greater accuracy in quantifying data losses, and greater detail reporting the occurrence of events that disrupt TDM communications, is especially important for trouble analysis in long-distance circuits. The complex interactions that occur when the service provided to a customer involves multiple companies, during xe2x80x9cbridge and rollxe2x80x9d re-direction of calls, for example, are accompanied by an increase in the incidence of abnormal service interruptions of unknown origin.
In hybrid ATM/TDM networks voice traffic is, in part, carried on asynchronous packet-switched circuit. Terminal adapters (TA), operating in the ATM domain reformat the payload received from conventional TDM networks into packets. In the ATM domain, voice and data are entirely recoverable from TDM inputs having OOF errors of less than a frame. Unfortunately, using standard TDM error criteria that slight OOF timing error will be reported as a serious outage until framing is reset. OOF-based error reporting has been a blunt instrument for analyzing trouble in conventional TDM payloads. In the new hybrid ATM/TDM networks the instrument is even less useful as it swamps the TDM parity and redundancy test and other measures that reflect actual signal degradation at the interface between TDM and ATM systems.
The data rate at the payload inputs to ATM networks need not be fixed and the relative packet timing across an ATM call is highly variable. There are three principal sources of delay in ATM communications: queuing delays in the packet switches; digital voice compression coding/decoding delays; and voice-packet assembly delays. In particular, the quality of a voice signal transmitted across an ATM network is sensitive to round-trip delay and packet-order error, but not to xe2x80x9cframing errorsxe2x80x9d or even individual dropped packets. Thus, the Terminal Adapters on the boundary between ATM and TDM networks that report OOF incidents as call outages do not accurately describe the frequency or extent of any impairment of the voice signal received by the ATM portion of the network, nor by the customers using the hybrid networks.
The present invention provides constant-frame error detection with synchronization criteria to provide a more precise signature of a circuit impairment.
In accordance with the present invention, a sub-second circuit impairment and framing-error profile is produced for a framed signal by a profiler having a bit counter, a frame pattern detector, a sync circuit and a profile extractor. Each time the frame pattern detector detects a frame in the framed signal, it produces a detected frame signal that resets the bit counter. The bit counter then begins to count the number of bits in the framed signal again. When the count in the bit counter reaches the number of bits expected in a frame of the framed signal it produces an estimated frame signal. If the estimated frame signal and the detected frame signal do not coincide with each other, a framing-error signal is produced.
Preferably a default frame signal is generated by a sync circuit to maintain accurate synchronization of detector and received signal when it happens that no bits are detected at the time when a detected frame signal is expected and at a time period calculated as a running average of the time between recent detected-frame signals has also expired without either a detected frame or an estimated frame signal. Because this pseudo-synchronous default signal does not coincide with a detected frame signal, a framing-error signal is produced.
In one embodiment, the detected frame signal and the bit count expected in each frame are also used to test data integrity. If the data (that is the number of bits expected in a frame after each detected frame begins) fails the test, a data error signal is produced.
Preferably, the timing of each incidence of an error signal is recorded by an identifier and that identifier is stored in a profile. When the error signal is a data error signal, the timing is preferably identified by a time-stamp value.
The output of the device of the present invention is a binary code message that establishes the format of the xe2x80x9cprofile.xe2x80x9d This message can be truncated when no impairments are detected.
The invention provides a compact sub-second error profile capable of resolving multiple sub-second error events so that the operation of high-speed signals, such as those in T1 and T3 networks, and synchronous optical networks (SONET) and their interactions with signals in other networks, can be accurately described, even when the T-carrier signal goes OOF. Sub-second accuracy permits trouble profiles built from bit-error data to be economically communicated across networks and analyzed to improve transmission quality and reliability. It also permits the technician to distinguish OOF timing errors from dropout or noise events causing data corruption. This sub-second accuracy is important for defending service reliability in ATM/TDM networks. In TDM networks it is particularly important where customer service, end-to-end, requires the facilities of multiple service providers. The resulting increase in abnormal disruptions from xe2x80x9cunknownxe2x80x9d sources requires detailed failure analysis to assure proper accountability.